Many unusual circles have appeared around the Croatian islands, causing the residents to worry. Some even say its aliens that cause it, because they often see circle formations in the sky above the location the circle are in the water. The circles are evenly spaced apart and are found around several islands. SCW

Like Croatia states:

During last year’s routine underwater mapping, 28 perfect round shapes were discovered in the underwater vicinity of Dugi Otok Island. Each of them has 50 meters in length, and the circles are 300 meters away from each other. They consist of unexpectedly rough terrain, devoid of any floral life, as if some Godzilla-like being walked the Adriatic’s bottom, leaving its circular footsteps in the underwater grassland.

"At first we thought it was some sort of photo manipulation, so we dived at one of the locations, and sure enough, the circles were there. They had a perfect edge, as if someone pulled the Posidonia out with a corkscrew."